The present invention relates to a transmission control system of an infinitely variable belt-drive automatic transmission for a vehicle, and particularly to a system which provides engine braking in the most proper mode at any vehicle speed when an active drive range is selected.
U.S. Pat. No. 4,369,675 discloses a control system for an infinitely variable belt-drive transmission providing an active drive mode. According to the conventional art, the transmission ratio is controlled in accordance with the depression of an accelerator pedal of a vehicle and with the engine speed which is represented by pitot pressure. The relationship between the engine speed and vehicle speed in accordance with the transmission is shown in FIG. 5, in which a line "l" represents the variation dependent on the largest transmission ratio, and M.sub.1 shows a transmission ratio varying line at the highest engine speed. When engine speed decreases, the engine speed and vehicle speed decrease along the smallest transmission ratio line "h" and the vehicle speed only decreases along a line M.sub.2 of the lowest engine speed. Such a transmission ratio variation pattern does not provide a large engine braking effect on the vehicle, which means an inactive drive mode.
In order to enable the transmission to operate so as to provide an active drive mode, an improvement is proposed, having a device for operating a transmission ratio control valve. In accordance with the improved system, when the active drive mode is selected at a point Ds in FIG. 5, engine speed rises to a line M.sub.3, which means an increase of the transmission ratio. Thus, the vehicle speed decreases along the line M.sub.3 with an increase of the transmission ratio, providing a high engine braking effect.
However, in such a system, the line M.sub.3 is set at to a certain level irrespective of the variation of vehicle speed. Accordingly, if the level is set at a high engine speed so as to provide a sufficient engine braking effect in a high vehicle speed range, extremely high engine braking acts on the vehicle at a low vehicle speed, causing unpleasant and unsmooth driving of the vehicle.